The present invention relates to an image forming material comprising a thermal transfer sheet and an image-receiving sheet, which can be used for a multicolor image formation method using laser light.
In the field of graphic art, an image is printed on a printing plate using a set of color-separation films prepared from a color original by using lithographic films. In general, a color proof is manufactured from the color-separation films before the main printing (i.e., actual printing operation) so as to check for errors in the color separation process or whether color correction or the like is necessary. The color proof is required to realize high resolution for enabling the formation of a halftone image with high reproducibility and to have capabilities such as high process stability. Furthermore, in order to obtain a color proof approximated to an actual printed matter, the materials used for the actual printed matter are preferably used for the materials of the color proof, for example, the substrate is preferably printing paper and the coloring material is preferably a pigment. With respect to the method for manufacturing the color proof, a dry process of using no developer solution is highly demanded.
Accompanying recent widespread use of computerized systems in the pre-printing process (in the pre-press field), a recording system of producing a color proof directly from digital signals has been developed as the dry preparation method of a color proof. These computerized systems are configured particularly for the purpose of producing a color proof having high image quality and by these systems, a halftone image of 150 lines/inch or more is generally reproduced. In order to record a proof having high image quality from digital signals, laser light capable of modulating by the digital signals and sharply focusing the recording light is used as the recording head. Accordingly, the recording material used with the laser is required to exhibit high recording sensitivity to the laser light and high resolution for enabling the reproduction of high definition halftone dots.
With respect to the recording material for use in the transfer image formation method utilizing laser light, a heat-fusion transfer sheet is known, where a light-to-heat conversion layer capable of absorbing laser light and generating heat and an image-forming layer containing a pigment dispersed in a heat-fusible component such as wax or binder are provided in this order on a support (see, JP-A-5-58045 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)). According to the image formation method using this recording material, heat is generated in the region irradiated with laser light of the light-to-heat conversion layer and the image-forming layer corresponding to the region is fused by the heat and transferred to an image-receiving sheet stacked and disposed on the transfer sheet, whereby a transfer image is formed on the image-receiving sheet.
JP-A-6-219052 discloses a thermal transfer sheet where a light-to-heat conversion layer containing a light-to-heat converting substance, a very thin (0.03 to 0.3 xcexcm) thermal release layer and an image-forming layer containing a coloring material are provided in this order on a support. In this thermal transfer sheet, upon irradiation with laser light, the bonding strength between the image-forming layer and the light-to-heat conversion layer bonded through the thermal release layer is weakened and a high definition image is formed on an image-receiving sheet stacked and disposed on the thermal transfer sheet. The image formation method using this thermal transfer sheet utilizes so-called xe2x80x9cablationxe2x80x9d, more specifically, a phenomenon that a part of the thermal release layer in the region irradiated with laser light is decomposed and vaporized and thereby the bonding strength between the image-forming layer and the light-to-heat conversion layer is weakened in that region, as a result, the image-forming layer in that region is transferred to an image-receiving sheet stacked on the thermal transfer sheet.
These image formation methods are advantageous in that a printing paper having provided thereon an image-receiving layer (adhesive layer) can be used as the image-receiving sheet material and a multicolor image can be easily obtained by sequentially transferring images of different colors on the image-receiving sheet and also in that a high definition image can be easily obtained. Therefore, these methods are useful for the production of a color proof (DDCP (direct digital color proof)) or a high definition mask image.
In such a multicolor image formation method, the light-to-heat conversion layer or image-forming layer of the thermal transfer sheet is formed by a coating method. The image-receiving layer of the image-receiving sheet is also formed by a coating method.
In order to obtain stable transferability (sensitivity), the image-forming layer or image-receiving layer must be uniformly coated. If such a layer is not uniformly coated, an uneven surface results or unevenness is generated in the surface energy or adhesive strength and this adversely affects the uniform transferability to the image-receiving sheet.
An object of the present invention is to provide a multicolor image forming material comprising a thermal transfer sheet and an image-receiving sheet, where at least either one of the image-forming layer and the image-receiving layer has a uniform coated surface state and the transferability of an image formed on the image-forming layer to the image-receiving sheet is improved.
According to the present invention, an image forming material having the following constitutions is provided and thereby the above-described object of the present invention can be attained.
1. A multicolor image forming material comprising an image-receiving sheet having an image-receiving layer and at least four thermal transfer sheets differing in color each having at least a light-to-heat conversion layer and an image-forming layer on a support, which is used for recording a multicolor image by superposing the image-forming layer of each thermal transfer sheet and the image-receiving layer of the image-receiving sheet to face each other, irradiating laser light and transferring the region irradiated with the laser light of the image-forming layer onto the image-receiving layer of the image-receiving sheet,
wherein at least either one of the image-receiving layer and the image-forming layer contains, as a fluorine-containing surfactant, a copolymer (I) comprising the following repeating units (A), (B) and (C): 
wherein
n represents an integer of 1 to 10,
x, y and z represent the molar fractions (%) of the repeating units (A), (B) and (C), respectively, and x is from 10 to 80%, y is from 5 to 85% and z is from 5 to 85%, provided that x+y+z=100 mol %,
s represents an integer of 2 to 18,
t represents an integer of 2 to 18,
PO represents xe2x80x94CH2CHCH3Oxe2x80x94, and
EO represents xe2x80x94CH2CH2Oxe2x80x94.
2. The multicolor image forming material as described in 1 above, wherein the resolution of the image transferred onto the image-receiving layer is 2,000 dpi or more.
3. The multicolor image forming material as described in 1 or 2 above, wherein the ratio (OD/film thickness) of the optical density (OD) to the film thickness of the light-to-heat conversion layer of each thermal transfer sheet is 0.57 or more.
4. The multicolor image forming material as described in any one of 1 to 3 above, wherein the ratio (OD/film thickness) of the optical density (OD) to the film thickness of the image-forming layer of each thermal transfer sheet is 1.80 or more.